Testing high frequency signals on a trace

ABSTRACT

A system, apparatus and method for testing and measuring high frequency signals on a trace is described. In one embodiment of the invention, a footprint is manufactured on a trace to allow the testing of a signal while reducing the amount of distortion caused by prior art structures and methods. The footprint is designed to reduce stub effects and capacitance on a signal being communicated on the trace.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

This application is a divisional application of and claims priority toU.S. patent application Ser. No. 11/305,749, entitled “Testing HighFrequency Signals on a Trace,” filed Dec. 16, 2005.

BACKGROUND

A. Technical Field

This invention relates to signal monitoring, and more particularly, totesting high frequency signals on a trace.

B. Background of the Invention

The importance of integrated circuitry is well known. Technologicaladvancements have led to continual reduction in the size of integratedcomponents and circuitry. The electronic devices employing integratedcircuitry have not only seen a reduction in their size but also animprovement in signal processing efficiency. One reason for theimprovement of signal processing is the use of higher frequency signalsthat are able to communicate large amounts of data within an integratedcircuit.

As integrated circuits have become smaller and signal frequencies withinthe circuits have increased, the ability to effectively test and measuresignals and components within the circuits has become more difficult.For example, it may be difficult for an engineer to locate a failure inan integrated circuit because of the problems in tapping or extractingan electrical signal internal to the integrated circuit. Theever-increasing signal frequencies on IC traces have made it difficultto effectively tap and monitor these internal IC signals.Electromagnetic interferences, including signal reflection anddistortion, oftentimes render a tapped signal unusable for monitoringpurposes. Furthermore, footprints on a trace that are designed to allowtesting of a signal may reduce the performance of the trace byeffectively creating a stub on the trace and/or adding unwantedcapacitance.

The existing methods of monitoring signals typically include the use ofan oscilloscope to probe a signal on a particular trace. FIG. 1illustrates an exemplary signal monitoring method using an oscilloscope.As shown therein, a driver 102 sends a signal to a receiver 104 througha signal trace 106, which acts as a medium through which the signaltravels. A metallic end of a scope probe 108 is brought in contact withthe signal trace 106 resulting in a portion of the signal to be divertedonto the probe and sent to the oscilloscope. The signal traversingbetween the driver and the receiver can thus be tested.

The probe 108 may distort the signal because its metallic contact mayfunction as a stub resulting in added dispersion and reflection to thesignal being monitored. If sufficiently high frequency signals are beingmonitored, this dispersion and reflection on the signal may render anoscilloscope reading of the signal to be imprecise or unusable.

A sub-miniature A (“SMA”) connector may be used to more effectivelymeasure a high speed signal. As illustrated in FIG. 2, “zero ohm”resistors R1 210 and R2 212 may be placed on the trace to allowconnection of the SMA connector. It is important to note that a zero ohmresistor may in fact have a small amount of resistance associated withit. During the normal operation of a driver 202 and a receiver 204, theresistor R1 210 is placed in the footprint area specified, while theresistor R2 212 is physically removed. To observe the signal on the PCBtrace 206 the resistor R1 210 is then removed and R2 212 placed in thefootprint area specified. The signal is directed towards SMA connector208 to be observed on the scope.

This method reduces the reflections but still adds stub and extracapacitance that are caused by the surface mount footprint on the trace.This stub and capacitance may lead to inaccurate results and distort thesignal received by the receiver 204. At specified frequency ofoperation, which tends to be more than 1 Gbps, the accuracy of theobserved results greatly affects the troubleshooting procedure. Theextra trace on board in mentioned operating conditions adds to theinaccuracy.

There is a need of a method designed to allow observance of highfrequency signals on trace without adding reflections and dispersionsinto the original signal.

SUMMARY OF THE INVENTION

The present invention provides a system, apparatus and method formeasuring and testing high frequency signals on a trace. In oneembodiment of the invention, a footprint is manufactured on a trace toallow the testing of a signal while reducing the amount of distortioncaused by prior art structures and methods. In one embodiment, thefootprint allows surface mount resistors to be connected to define aparticular path for the signal. A first path is provided in which thesignal is communicated on the trace between IC components, such as adriver and a receiver. A second path is provided in which the signal iscommunicated from the trace to a testing or measurement device, such asan oscilloscope.

In one embodiment of the invention, the footprint is manufactured bypartially overlaying a first surface mount area with a second surfacemount area. The first and second surface mount areas may be positionedat a 90 or 180 degree angle from each other. This footprint designminimizes stub effects and capacitance generated by prior art testingconnections.

A user is able to define the path a signal travels by removing orinserting surface mount components on the footprint. In particular, thefootprint may operate in two modes depending on where surface mountcomponents are located on the footprint. These surface mount componentsare components that conduct electrical current such as zero ohmresistors and alternating current coupling capacitors.

In one embodiment, the footprint is provided to test single endedsignaling. In another embodiment, the footprint is provided to testdifferential signaling. Various testing devices may be used inconnection with the footprint including an oscilloscope with SMAconnector(s).

Other objects, features and advantages of the invention will be apparentfrom the drawings, and from the detailed description that follows below.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to embodiments of the invention, examples ofwhich may be illustrated in the accompanying figures. These figures areintended to be illustrative, not limiting. Although the invention isgenerally described in the context of these embodiments, it should beunderstood that it is not intended to limit the scope of the inventionto these particular embodiments.

FIG. 1 shows a block diagram of a system for directing signal formonitoring using scope probe.

FIG. 2 is a block diagram of a system-allowing signal monitoring presentin prior art.

FIG. 3 is a block diagram of the system as designed by the presentinvention to allow monitoring of signal using single ended signalingaccording to one embodiment of the invention.

FIG. 4 is a diagram representing the footprints design as suggested bythe present invention using differential signaling according to oneembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A system, apparatus and method for observing a high frequency signal ona trace is described. The system may operate in multiple modes includinga first mode in which a signal is communicated between a driver and ahost, and a second mode in which the signal is communicated to ameasurement device, such as an oscilloscope. A footprint that is locatedon a trace allows for the switching between modes while minimizing theamount of distortion on the signal during either of the modes ofoperation.

The invention described herein is explained using specific exemplarydetails for better understanding. However, the invention disclosed canbe worked on by a person skilled in the art without the use of thesespecific details. The implementations of the invention can be embodiedinto a multiple types of printed circuit boards. The block diagramsshown are only exemplary implementation as per the rules dictated by theinvention. Also, the connections between various components may notnecessarily be direct. The components may not necessarily be on sameboard or plane but may be connected using a backplane. Further, thesignal routing in between can be subjected to encoding, re-formatting ormodifications.

References in the specification to “one embodiment” or “an embodiment”means that a particular feature, structure, characteristic, or functiondescribed in connection with the embodiment is included in at lest oneembodiment of the invention. The appearances of the phrase “in oneembodiment” in various places in the specification are not necessarilyall referring to the same embodiment.

A. System Overview

FIG. 3 illustrates a signal measurement footprint located on a traceaccording to one embodiment of the invention. The signal being observedis the signal traversing between the driver 302 and the receiver 304 ona common board. The components can alternatively be on different boardsconnected using a backplane or other connecting means. The signaltravels from driver to the receiver via trace 306, such as a PCB trace.

According to one embodiment of the invention, the system has two modesof operation. During a first mode of operation, the signal iscommunicated from a driver 302 to a receiver 304 via the trace 306.Comparatively, during a second mode of operation, the signal iscommunicated from the driver 302 to a connector 308, such as an SMAconnector, so that it may be measured by a measuring device. Theconnector 308 may be defined to have specific impedance value, aligningit with the input of the scope. For example, an SMA connector may beused having an impedance value be 50 ohms that matches an impedancevalue of an oscilloscope.

A footprint is provided on the trace to allow a user to switch betweenthe two modes. The footprint comprises a first area 310 a on which asurface mount component may be positioned to enable current to flowbetween the driver 302 and the receiver 304. The footprint comprises asecond area 310 b that is partially overlaid on the first area 310 a andon which a surface mount component may be positioned to enable currentto flow from the driver 302 to the connector 308. It is important tonote that the driver 302 and receiver 304 are exemplary components andthe present invention is applicable to any two components between whicha signal travels.

A user is able to effectively switch the mode of the footprint byremoving or inserting a surface mount component on the first or secondareas of the footprint. The surface mount components may be numerousdifferent components that conduct an electrical signal including a zeroohm resistor and alternating current coupling capacitor.

B. Footprint Design

The invention minimizes the distortion of a signal using a specificfootprint located on a trace. Referring again to FIG. 3, a first surfacearea 310 a and a second overlaid surface area 310 b create a footprintthat allows a user to switch between modes in order to drive a receiver304 or test the signal going to the receiver 304. In one of theembodiment of the invention, the first and second surface areas 310 a,310 b are positioned at 90 degrees or 180 degrees relative to eachother. The angle between the two footprints may be changed depending onthe design of the trace. However, the preferred angular displacementbetween the first and second surface areas 310 a, 310 b is 90° and 180°.Such an arrangement provides overlapped resistor/footprint area, whichminimizes any stubs and capacitance that might have been caused by thefootprint.

In another of the embodiment of the invention, differential signalingmeasurement is provided by multiple footprints on a trace. FIG. 4illustrates exemplary footprints that provide differential measurement asignal on a trace. The footprints 406, 408 allow for differentialsignals to be measured using a connector(s) that interfaces with thefootprints. The footprints 406, 408 comprise overlaid surface areas 406a, 406 b and 408 a, 408 b respectively, minimizing any capacitance orstubs caused by the footprints.

C. Modes of Operation

As described above, one embodiment of the invention provides for a tracesignal measurement system having two modes of operation. A first modeoperates in which a signal is communicated on a trace without anymeasurement and a second mode operates in which the signal iscommunicated to a measuring device, such as an oscilloscope. A footprintis provided on the trace that allows a user to switch between modes.

a) First Mode of Operation

Referring to FIG. 3, a surface mount component is connected on a firstsurface area 310 a of the footprint 310 and a second surface area 310 bon the footprint 310 is open during operation in the first mode. Thissurface mount component may be a variety of different components thattransmit electrical current including a zero ohm resistor or capacitor.Accordingly, current is allowed to flow between the driver 302 and thereceiver 304 while prevented from flowing through the second surfacearea 310 b. The footprint 310 minimizes or removes a stub andcapacitance effects resulting from the gap at the second surface area310 b.

FIG. 4 illustrates a footprint or footprints that enable differentialtesting of a signal according to one embodiment of the invention. Inthis particular example, a first footprint 406 and a second footprint408 are located on a trace. The first footprint 406 has a first surfacearea 406 a and a second surface area 406 b similar to the footprintdescribed above. The second footprint 408 has also a first surface area408 a and a second surface area 408 b.

A user may switch to a first mode, in which the signal travels betweentwo components coupled by the trace, by inserting surface mountcomponents at surface areas 406 a and 406 b. Accordingly current isallowed to flow through the surface areas 406 a and 408 a with minimaldistortion caused by the design of the footprints 406 and 408. Currentis not able to flow through the surface areas 406 b and 408 b because ofthe gap left by the removal of surface mount components. Thedifferential mode signaling may improves electromagnetic compatibilityand may be more suited to the high frequency applications.

b) Second Mode of Operation

A second mode of operation is provided that allows a signal on a traceto be measured and otherwise tested. Referring to FIG. 3, a surfacemount component is connected at surface area 310 b on footprint 310while a gap at 310 a is created by removing a surface mount component.Once again, these surface mount components may be a variety of differentcomponents that transmit electrical current including a zero ohmresistors or capacitors. Attaching a component on footprint area 310 ballows the signal from the driver 302 to be directed to a connector 308,such as an SMA connector, on which the signal is provided to a measuringdevice.

Referring to FIG. 4, a second mode of operation is shown relative to adifferential signaling model. The second mode of operation is initiatedby removing the surface mount components at 406 a and 408 b andinserting surface mount components at 408 a and 408 b. In oneembodiment, surface mount capacitors are used because they will transmithigh frequency signals and filter lower frequency signals. A connectoror connectors are coupled to transmit the signal to a measurementdevice, such as an oscilloscope.

During the second mode of operation, the signal being tapped may betransmitted on a uniform trace, having a minimum excess trace structurethat may function as a stub or provide unwanted capacitance.

The present invention thus provides a method for high accuracy andminimum distortion monitoring of a signal as is needed in high frequencymeasurements. The method can be used during testing, troubleshooting andvalidation procedures to ensure proper working of the electronicdevices.

While the present invention has been described with reference to certainexemplary embodiments, those skilled in the art will recognize thatvarious modifications may be provided. Accordingly, the scope of theinvention is to be limited only by the following claims.

1. A method for testing a signal in an integrated circuit, the methodcomprising: removing a first surface mount component from a first areaof a footprint in the integrated circuit to prevent the signal frombeing transmitted on the trace; inserting a second surface mountcomponent on a second area of the footprint to communicate the signal toa measuring device; wherein the second area of the footprint is locatedat a ninety degree angle relative to the first area of the footprint;and measuring the signal using the measuring device that received thesignal.
 2. The method of claim 1 wherein a second footprint is providedto enable testing a differential signal.
 3. The method of claim 1wherein the second surface mount component is a zero ohm resistor. 4.The method of claim 1 wherein the second surface mount component is analternating current coupling capacitor.
 5. The method of claim 1 furthercomprising the step of coupling a connector to the second surface mountcomponent and the measuring device.
 6. The method of claim 5 wherein theconnector is a sub-miniature A connector.
 7. The method of claim 5wherein the measuring device is an oscilloscope.